1. Why Make Condition Monitoring ?
Longer time between overhauls
Reduced repair duration
Reduction of spare-part stock
Less unexpected breakdowns
Elimination of secondary damage
Reduction in business interruption and damage insurance premiums
Selection and purchase of instrumentation
Initial investigations, selection of monitoring points, establishment of limits
Training
Yes No
Condition Monitoring

2. Predictive Maintenance
Types of Maintenance
Production
Maintenance
Run to Breakdown
Time
Production
Production
Condition Based
Predictive Maintenance
Time
Intermittent or
continuous
measurements
Maintenance methods used in industry can broadly be categorized into three: Run to Breakdown, Time-based Preventive Maintenance, and Predictive Maintenance.
Run to Breakdown
In industries running many inexpensive machines and having each important process duplicated, machines are usually run until they break down. Loss of production is not significant as spare machines can usually take over.
In few cases, large unduplicated process-machines are still run to breakdown. In this case it is of vital importance to know what is going wrong and when breakdown is likely to occur. This information can be obtained by studying vibration spectra trends built up from regular measurements. Knowing what is going wrong will allow the plant engineer to order the necessary spare parts ahead of expected breakdown and thereby avoid a large standing stock of spare parts. Furthermore, maintenance personnel are better prepared and can be expected to effect a more reliable repair in a shorter time.
Predictive Maintenance
A reference measurement is taken when the machine is new, newly refurbished or found working in good conditions. Vibration measurements are taken continuously or at regular intervals, and compared against the reference level. If an increase of vibration level at any frequency has exceeded a pre-defined alert level, the machine is monitored more frequently and maintenance is scheduled. Often a trend is calculated based on the vibration data and the maintenance is scheduled according to the alarm level.
Trend
Measurement
value
Alarm
Alert
Time

3. Time based preventive maintenance
Failure rate
If is not broken,
don't fix it !
Where important machines are not fully duplicated or where unscheduled production stops can result in large losses, maintenance operations are often performed at fixed intervals such as every 3000 operating hours or once per year. This system is therefore called Preventive Maintenance or more correctly Time-based Preventive Maintenance.
The Service intervals are often determined statistically as the period measured from the time when the machines are in a new or fully serviced condition, to when the manufacturer expects no more than 2% of the machine population to have failed.
By servicing at these intervals it is generally believed that, as 98% of the machines should survive the running period, failure should be a rare occurrence.
Experience as shown that in the vast majority of cases time-based preventive maintenance is uneconomical. A significant fact is, that the failure rate of many machines is not improved by replacing wearing parts regularly. On the contrary, the reliability of newly serviced machines is often reduced temporarily by human interference.
As the actual failure pattern for each individual machine cannot be predicted, time-based preventive maintenance cannot be efficiently applied. An individual approach is needed, and this is the axiom of Condition - Based (Predictive) Maintenance.
Time
Maintenance at regular intervals

4. Protective Systems For avoiding sudden catastrophic breakdown
Continuous measurements
Simple measurements
Relays for automatically tripping.
For safeguarding important machinery for catastrophic breakdowns due to sudden changes, safety monitoring is often applied.
A shut down relay will trip the machine as soon as the vibrations pass a preset value.
Shutdown
Relay
Annunciation
Relay

5. What to Measure Vibration Temperature Oil Analysis Fluid Pressure
Gives very early warning
Most faults, Unbalance, Misalignment, Bearing
faults, etc..
Temperature
Inexpensive transducers
Trust bearing problems.
Oil Analysis
Reciprocating machinery
Combined with Vibration
Several measurements can be performed for for assessing the state of a machine.
Vibration measurements are giving the best overall machine condition on rotating machinery.
With vibration measurements it is possible to predict machine faults months even years before developing into machine failures.
Fluid Pressure
Process monitoring
No early warning

6. Different faults Vibration amplitude Frequency
The vibration that is measured on this machine is analyzed in a frequency spectrum. Each frequency peak corresponds to a cyclic event in the machine.
The leftmost peak in the spectrum corresponds to the rotation speed of the fan.
The second peak to the motor rotation frequency. Coupling misalignments are often coming up at two times this frequency.
At the frequency of tooth meash (and its harmonics), you may see gear problems. In the high frequency you may see problems in rotating element bearings.

7. Where are vibrations coming from ?
Internal
Forces
The key to a good approach is the understanding that for maintenance, we are not actually interested in vibration. We really want to find out how the condition of the machine is changing i. e. how the internal forces in the machine are changing with time and accelerating the breakdown of components.
Changes in forces can be caused by direct changes in the work process or by changes in the properties of the machine elements.
The amount of vibration generated by the forces passing through a machine structure at any point is given by Force * Mobility = Vibration. (The mobility is a measure of the willingness of the structure to be set into motion and is also the inverse of the mechanical impedance.)
The vibrations can either be measured as relative movement of the shaft with respect to the bearing housing using a displacement probe (left bearing on the drawing) or as absolute movement of the bearing housing using an accelerometer.
Structural
Mobility
Vibration =Force Mobility

8. Force Mobility Vibration
The mobility may be different at different frequencies. This means that the highest peak in the vibration spectrum does not necessarily correspond to the highest force component in the machine.
RPM
Forces * Mobility = Vibration D
The mobility is also very dependent upon where on the machine you make the measurement.
The mobility of a structure can be measured by exiting it with a force (measured with a force transducer), from a hammer or a shaker and measure the produced vibration.
We will see that the mobility is very is varying strongly from point to point.
Therefore the vibration measured may vary very strongly from point to point.
(the following two pages are omitted please go to page 11)

9. Measuring Mobility on simple structures
Force Input
Output
Impulse
response
function
Mobility
Point 1 f
H(f)
h(t)
Point 2

10. Vibrations may be different from point to point
RPM
Forces * Mobility = Vibration
Point 2

11. Vibration standard guidelines
Not
Permissible
Good
Large Machines
with rigid and heavy
foundations whose
natural speed exceeds
machine speed
Just Tolerable
Allowable
Small
Machines< 15 kW
Just
Tolerable
15 kW<
Medium Machines
<75kW
<300 kW on special
foundations
2.5 times = 8dB
10 times = 20dB
Group K
Group M
Group G 45 28 18
11.2
7.1
4.5
2.8
1.8
1.12
1.71
0.45
0.28
0.18
Velocity mm/s RMS
ISO ( BS 4675 , VDI 2056 )
The standards can be used as guidelines, when specifying new machines, or when no reference measurements exists.
We note that going from one that the change between two classes corresponds to 2.5 times change. And a change from good to not permissible corresponds to a 10 times change.

12. Create a reference for each measurement point
first measurement = reference
alert limit
Danger limit
10 times
2.5
times
Instead of using an overall criteria, machines should be judged independently taking an actual measurement as reference.
The alert limit is normally set times (6 to 8 dB) above the reference.
The alarm limit ( or trend limit) is set 10 times above the reference, corresponding to the fact that most machines are designed with a 10 times safety factor for dynamic forces.
The alert and danger limits are widened in order to allow for speed variations of the machine.

13. Trending
It is important when making a trend to select the frequency range where the fault is developed.
Furthermore the trend should be calculated only in the time range where the fault is actually developing. This can be done by excluding earlier measurements from the trend calculation.

14. Making analysis gives earlier warning
Overall Level L f
By using frequency analysis in stead of overall vibration criteria when analyzing vibrations, we obtain a much earlier warning of upcoming problems.

15. Frequency analysis Time Frequency (Period) 2:00 every 0:20 2:20 2:40
Same
information
Time
2:00
2:20
2:40
3:00
3:20
3:40
4:00
4:20 …
Frequency (Period)
every 0:20 11 12 1 10 2 9 3 8 4 7 6 5
Phase
First 2:00
What is frequency analysis ?
Lets look at the two time tables.
On the left table we have listed all the departures of the trains from the station. The same information can be attained by stating that there is a train every 40 hours ( frequency or period). If we add the information that there is a train at 2 o’clock ( phase) we have the same information on the in the left table as the two statements to the right.
The two statements to the right are however shorter, more convenient and easier to remember than the whole time table.

16. Shorter time cycle, Higher Frequency
Time Signal Frequency Spectrum
p(t) A t T f
1/T
p(t) A
We see that the longer the period of the sine wave, the lower the frequency.
The magnitude of the peak in the spectrum corresponds to the energy content of the sine wave (RMS). t t f
1/t

17. Spectrum Analysis
The information in the time domain and the frequency domain is the same.

18. Acceleration Velocity Displacement
Vibration may be measured in acceleration, velocity or displacement.
The for higher frequencies the velocity is higher than the displacement,
and acceleration is higher than velocity.
Note that the peaks in the three spectra from a real machine are situated at the same frequencies.
For very high frequencies, the peaks may not be seen in the displacement spectra due to noise.
Time
(Simple vibration)
Frequency
(real machine)

19. Detector a For Sine waves only: RMS RMS peak T = averaging period
Crest factor =
K factor =
True peak - peak
The most commonly used quantity for expressing amplitudes is the RMS value or the Root Mean Square value. The RMS value is related to the energy content of the signal, and is therefore far more practical to use than for example the arithmetical average.
(For a Sine wave the RMS is 10 % higher than the arithmetical average)
Spectrum values are nearly always RMS. In the USA it has been praxis to scale the spectrum values to RMS peak which is the the RMS value times 1,41.
True peak and true peak -peak values are used for displacement measurements inside bearing, where the clearance has to be measured. Overload detectors are also measuring true peak values.
The crest factor and the K factor are use full for detection of defects in rolling element bearings. These defects start with very short peaks with very little energy.
RMS
RMS
peak

20. Classification of Machines
Driving Machines
Electric Motors
Engines
Diesel
Gas
Turbines
Steam
Water
Hydraulic
Wind
Intermediate
Couplings
Flexible
Fixed
Cardan
Gears
Helical
Straight
Planetary
Belts
Driven Machines
Pumps
Compressors
Generators
Material Transportation
Paper Machines
Transport Rollers
Other
Steel Roller
Ship Propellers
Vehicles
etc.
Rotating machines consists of different components.
Each component has its own potential failure mode. The potential failure mode are accompanied by its own special vibration characteristic.
In the following we shall see examples of different signals produced by rotating machinery.

21. Rolling Element Bearings
Types of Bearings
Journal Bearings
Stationary Signals
Relative Low Frequency
Displacement transducer
Rolling Element Bearings
Modulated Random Noise
Pulsating signals
High Frequency
Accelerometers
No machine component is as important for the potential failure mode as the bearing.
Two main type of bearings are used for rotating machinery.
Rotating element bearings and Journal bearings or sleeve bearings.

22. Stationary signals No frequency analysis irrelevant Temperature
Pressure
Axial Displacement of shaft
Radial displacement of shaft contains both:
Stationary
Dynamic Vibration (orbit & spectrum)
We will now look at different signal types.
Stationary signals or quasi stationary signals are slowly varying signals such as pressure (not sound) and temperature variations in machinery. The axial movement of a shaft is example of a quasi stationary signals. When radial movement is measured with a non contact displacement transducer the signal is divided into a stationary part and a vibration part.
There is normally no sense in making frequency analysis of stationary signals.
time

23. + = Harmonic signals t f All Periodic signals that are not
sinusoidal, contains harmonics
Examples:
Unbalance
Misalignment
Tooth meash in gear boxes
According to Fouriers theorem, all periodic signals can be produced from sinusoidals.
Often machine signals are not purely sinusoidal, therefore we most of the see harmonics in the machine signal spectrum. f

24. Random Signals f t Random signals produce spectra without lines
Most signals from machines is a combination of harmonic and random signals
Following machine defect produce random signals:
Cavitation
Lubrication Problems
Rolling Element Bearing faults
Bearing Mounting defect
Flow Exited
Machine vibration signals normally consists of a deterministic (harmonic) part and a random part. The random noise may have many origins within the machine. When measured as vibration it is often amplified by the structural resonances in the signal path.

25. Structural resonances
Impulse signals
Excitation of
Structural resonances t
f=1/t f
Impulsive signals may have many origins such as
incipient faults in rolling element bearings
Electrical discharges
Rubs due to inadequate clearance.
The impact will excite the resonances of the machine up to a frequency which is inversely related to the period length of the impact.
Example:
You need a hard impact to ring a bell, if you hit the a bell with something soft the impact time is to long to excite the resonance.
Rubs in seals and bearings
Rolling element bearing faults
Electrical discharges
Frequency Modulation
Amplitude modulation
Produce sidebands

26. Modulated Signals t 1/t t f 1X Tm 5 kHz Frequency Modulation
Torsional Load
Gearboxes:
Tooth Fatigue
Eccentricity
Frequency Modulation
Amplitude modulation
Produce sidebands
Sideband spacing = carrier wave period
Complex signals being frequency and/or amplitude modulated are often seen in gear boxes and machines having Torsional vibration problems.
Modulated signals produce side bands around the (high) carrier frequency in the spectrum where the side band spacing is corresponding to the (low) modulation frequency.
Cepstrum analysis is used for identifying the energy of side band families.

27. Machine Signal Types Semi Static Harmonic Modulated Random Pulsed Time
Shaft Position
Harmonic
Imbalance
Misalignment
Modulated
Torsional Load
Tooth Fatigue
Eccentricity
1X X Hz
1X kHz
Random
Lubrication Problems
Rolling Element (RE)
Bearing Mounting defect
Flow Exited
1X kHz
Pulsed
RE Bearing Wear
Rubs
Blade Damage, fouling
Surge, Cavitation,
Local Tooth Defects
1X kHz
Time
Frequency

28. B = Bandwidth T = Time B*T > 1
Averaging f n
B = Bandwidth
T = Time
B*T > 1
Measurements are averaged in order to smooth out varying amplitude components and to improve statistical accuracy.
The number of averages multiplied by the measurement time for a spectrum measurement should be chosen to be at least as long as 3 to 5 periods of the lowest frequency, beat and modulation frequencies included.
The product between he bandwidth of the analysis and the time for the analysis must be larger than one.
For FFT measurements the BT = 1 in the lowest frequency band. The BT value can be increased by increasing the number of averages.
The gives the statistical error for a given BT product.

29. Linear or Logarithmic Amplitude
Linear amplitude:
Only few peaks
can be seen
Logarithmic
amplitude:
Relative limits are easy seen
Small peaks
indicating potential
problems are
easily spotted
Using a logarithmic amplitude scale makes it possible to see development of small frequency components which may give early warning of a potential failure mode.
The logarithmic frequency scale also gives a better view of the alarm limits.